Add es.filter

[mariusGundersen]

There should be an equivalent of array.prototype.filter. I have used this simple wrapper around es.map:

es.filter = function(func){
  return es.map(function(data, next){
    func(data, function(result){
      if(result){
        next(null, data);
      }else{
        next();
      }
    })
  });
}

It can be used like this:

stream.pipe(es.filter(function(data, keep){
  keep(data.length > 0);
})).pipe(output);

[dominictarr]

I'm not adding any more features to this module, but this could be good as it's own module.